1. Field of the Invention
The present invention relates to a three-way bleed type proportional electromagnetic valve in which the fluid force that is proportional to the output pressure and the pressing force that is proportional to the energization current of a solenoid coil act on a bleed-valve element and the bleed valve element is displaced to a position where the two kinds of force are balanced with each other, whereby the output pressure is made proportional to the energization current.
2. Description of the Related Art
In hydraulic circuits of electronic control type automatic transmissions (hereinafter abbreviated as ATs) for automobiles, a bleed type proportional electromagnetic valve in which the output pressure is controlled so as to be proportional to the energization current is used to change the operating oil pressure of each operating portion of an AT.
First, a description will be made of a method for using a bleed type proportional electromagnetic valve in a hydraulic circuit of the AT. Automatic transmission fluid (hereinafter abbreviated as ATF) stored in an oil pan is sucked by an oil pump that is driven in synchronism with an engine. After its pressure is adjusted to a prescribed value by a regulator or the like, the ATF is compression-transported to the input port of each electromagnetic valve. The bleed type proportional electromagnetic valve can produce a prescribed output pressure by controlling the load imposed on the bleed valve element by controlling the current supplied to the solenoid coil in accordance with the automobile running state. A gear shift is effected by controlling the opening/closing of a control valve provided in the hydraulic circuit of the AT using the above output pressure. ATF that has passed the bleed valve element of the bleed type proportional electromagnetic valve is collected into the oil pan via an ejection port.
The flow rate of the oil pump, which is a gear pump or the like, is always set to a maximum necessary value. Since the oil pump discharges ATF at a maximum flow rate, the reduction of the energy consumption of the oil pump is an important factor in increasing the fuel efficiency.
The structure of the bleed type proportional electromagnetic valve is generally classified into two types by the relationship between the energization current and the output pressure. The first type is a normally high type (hereinafter abbreviated as “N/H type”) in which the output pressure is high in a non-energization state and decreases as the current increases. The second type is a normally low type (hereinafter abbreviated as “N/L type”) in which, conversely, the output pressure is low in a non-energization state and increases with the current.
FIGS. 11 and 12 are sectional views showing a conventional N/H-type, two-way bleed type proportional electromagnetic valve. As shown in FIGS. 11 and 12, a solenoid coil 2 is provided inside a cylindrical case 1 that defines a main body outward shape. The solenoid coil 2 has a terminal 3 and a connector 4 for its energization from an external power source. A core (fixed core) 5 and a yoke 6 for formation of a closed magnetic path are fixed to the respective ends of the case 1 by welding so as to house the solenoid coil 2. A housing 7 to be inserted into a valve body (not shown) of the hydraulic circuit of the AT is fixed to the yoke 6 by welding. The yoke 6 is provided with a bleed valve guide 6a that extends inward so as to taper.
The housing 7 is provided with an ATF input port 7a, output port 7b, and ejection port 7c. A valve seat 8 is press-fit in the housing 7, that is, in the flow passage connecting the input port 7a and the output port 7b to the ejection port 7c. The valve seat 8 is formed with a bleed valve seat portion 8a on its ejection port 7c side. A spherical bleed valve element 9 is loosely fit in the bleed valve guide 6a so as to be slidable. O-rings 10a and 10b are provided for sealing between the ports 7a–7c. The thus-configured bleed-type proportional electromagnetic valve is fixed to the valve body by bolts or the like via a flange (not shown) that is fixed to the housing 7 by welding.
A plunger 11 as a movable core is disposed inside the solenoid coil 2. A rod 12 is press-fit in the inner circumferential surface of the plunger 11 coaxially and hence is movable together with the plunger 11. The rod 12 is supported by, that is, loosely fit in, non-magnetic sliding bearings 13 and 14 located on both sides with the plunger 11 interposed in between. The one sliding bearing 13 is press-fit in the bleed valve guide 6a of the yoke 6 and the other sliding bearing 14 is loosely fit in the inner circumferential surface of the core 5.
To prevent an operation failure due to magnetic sticking of the core 5 and the plunger 11, an annular, non-magnetic stopper 15 is disposed around the rod 12 so as to be in contact with the end face of the plunger 11. A spring 16 for output pressure adjustment is disposed between the end face of the stopper 15 and the sliding bearing 14. A load adjusting member 17 such as a spring pin is press-fit in the inner circumferential surface of the core 5 so as to compress the spring 16 via the sliding bearing 14. In this state, the rod 12 is pressed via the stopper 15 and the plunger 11 and the yoke-6-side end face of the rod 12 presses the bleed valve element 9. As a result, the bleed valve element 9 rests on the bleed valve seat portion 8a and the valve is closed.
Next, the operation of the N/H-type, two-way bleed type will be described. First, in a state that the solenoid coil 2 is not energized as shown in FIG. 11, as described above the compressed spring 16 presses the end face of the plunger 11 via the stopper 15 and hence the rod 12, which is integral with the plunger 11, presses the bleed valve element 9 against the bleed valve seat portion 8a. A maximum output pressure is obtained when the output pressure of ATF flowing through the output port 7b after passing through the input port 7a and the housing 7 is balanced with the pressing force acting on the bleed valve element 9 from the rod 12 (i.e., the force from the compressed spring 16) divided by the area S (=π(φd)2/4; φd: diameter of the bleed valve seat 8) of the bleed valve seat 8. The maximum output pressure can be set in a range that it is lower than the input pressure by adjusting the force from the compressed spring 16 by adjusting the press fit length of the load adjusting member 17.
When the solenoid 2 is energized via the terminal 3, a magnetic field is generated and a closed magnetic circuit is formed by the case 1, the core 5, the plunger 11, and the yoke 6. As a result, magnetic attractive force is generated between the excited core 5 and the plunger 11 in the movable direction of the plunger 11. Since the magnetic attractive force acts against the force from the spring 16, the pressing force acting on the bleed valve element 9 from the rod 12 is decreased ((force from compressed spring 16)−(magnetic attractive force)). The individual parts are shaped so that the pressing force becomes proportional to the current independently of the position of the plunger 11 in its movable range. That is, when the current is constant, the pressing force is constant independently of the position of the plunger 11.
As a result, the bleed valve element 9 is separated from the bleed valve seat portion 8a and displaced to a position where the pressing force acting on the bleed valve element 9 from the rod 12 is balanced with the fluid force that is proportional to the output pressure at the output port 7b. As the current flowing through the solenoid coil 2 increases, the pressing force acting on the bleed valve element 9 from the rod 12 decreases and hence the output pressure also decreases. In a state that the output pressure is controlled to a minimum value, the input port 7a communicates with the ejection port 7c and hence part of the AFT flows from the input port 7a to the ejection port 7c. 
In an ordinary output pressure control, the magnetic attractive force is controlled by the current so as to be weaker than the force from the compressed spring 16 and hence the plunger 11 does not contact the core 5 via the stopper 15. However, if the current is so large that the magnetic attractive force is stronger than the force form the compressed spring 16, the stopper 15 that is attached to the plunger 11 is kept in contact with the core 5 as shown in FIG. 12.
FIGS. 13 and 14 show a conventional N/L-type, two-way bleed type proportional electromagnetic valve, which is approximately the same in configuration as the above N/H-type two-way bleed type proportional electromagnetic valve except for the following points. The core 5 and the yoke 6 are arranged in the opposite manner. Both of sliding bearings 18 and 19 are press-fit; in particular, the sliding bearing 19 is formed with a flange and thereby given a stopper function of stopping the plunger 11. The spring 16 for output pressure adjustment and the load adjusting member 17 are absent. The stopper 15 for preventing sticking of the core 5 and the plunger 11 is absent. Further, in a non-energization state, the bleed valve element 9 is separated from the bleed valve seat portion 8a by the fluid force that is proportional to the output pressure, whereby the valve is opened.
Next, the operation of this type of proportional electromagnetic valve will be described. In a state that the solenoid coil 2 is not energized (see FIG. 13), the fluid force that is proportional to the output pressure acts on the bleed valve element 9 and hence the bleed valve element 9 is separated from the bleed valve seat portion 8a: a minimum output pressure is obtained. Since the input port 7a communicates with the ejection port 7c, part of the AFT flows from the input port 7a to the ejection port 7c. 
When the solenoid 2 is energized via the terminal 3, a magnetic field is generated and a closed magnetic circuit is formed by the case 1, the core 5, the plunger 11, and the yoke 6. As a result, magnetic attractive force is generated between the excited core 5 and the plunger 11 in the movable direction of the plunger 11. The magnetic attractive force acts in such a direction as to move the bleed valve element 9 closer to the bleed valve seat 8, that is, pressing force (=magnetic attractive force) acts on the bleed valve element 9 from the rod 12. The individual parts are shaped so that the pressing force becomes proportional to the current independently of the position of the plunger 11 in its movable range. That is, when the current is constant, the pressing force is constant independently of the position of the plunger 11.
As a result, the bleed valve element 9 is displaced to a position where the pressing force acting on the bleed valve element 9 from the rod 12 is balanced with the fluid force that is proportional to the output pressure at the output port 7b. As the current flowing through the solenoid coil 2 increases, the pressing force acting on the bleed valve element 9 from the rod 12 increases and hence the output pressure also increases. A maximum output pressure is obtained when the pressing force is stronger than the input pressure multiplied by the area S (=π(φd)2/4; φd: diameter of the bleed valve seat 8) of the bleed valve seat 8 and hence the bleed valve element 9 rests on the bleed valve seat portion 8a (the valve is closed). FIG. 14 shows this state.
As described above, in each of the N/H-type valve and the N/L-type valve, a state that the input port 7a and the ejection port 7c communicate with each other occurs when the output pressure is controlled to the minimum value. Therefore, ATF flows from the input port 7a to the ejection port 7c, which increases the necessary flow rate of the oil pump for giving input pressure to the input port 7a and hence increases the size of the oil pump. This results in a problem that the energy that is consumed by the oil pump is increased. To solve this problem, a three-way bleed type proportional electromagnetic valve has been proposed as disclosed in Japanese patent publication JP-A-2002-286152.
In the valve disclosed in the patent publication, when the output pressure is controlled to a minimum value, a state that the input port and the ejection port are isolated from each other and a state that the output port and the ejection port communicate with each other are established, whereby ATF is prevented from flowing from the input port to the ejection port. However, the valve of patent document-1 employs a structure that a stop valve element (ball valve element 24) can contact and be separated from a bleed valve element (composed of a bleed valve element portion 3 and a rod portion 4) for controlling the output pressure. In particular, the stop valve element is a spherical poppet valve (ball valve element 24). Therefore, force acts on the ball valve element because of a pressure of AFT flowing around the ball valve element and only the axial component (i.e., the component toward the bleed valve element) of that force serves as a load that is imposed on the bleed valve element.
As a result, the flow of ATF around the ball valve element is unstable depending on the flow rate of the AFT and the oil passage shape and the pressure distribution on the surface of the ball valve element every moment. Therefore, the force acting on the ball valve element is also unstable and its axial component influences the behavior of the bleed valve element, resulting in a problem that the output pressure and flow rate characteristics are unstable. To solve this problem, it is necessary to stabilize the axial component of the force acting on the ball valve element. However, much time is needed to optimize the oil passage shape etc., which increases the development cost.